1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor optical device, and more particularly to a method for manufacturing a semiconductor optical device in which the waveguide ridge has an electrode on its top.
2. Description of the Related Art
There has been a need for emission of light in the blue to ultraviolet wavelength range to enhance the recording density of optical discs. In order to meet such a need, intense R&D effort has recently been carried out to develop nitride semiconductor lasers formed of a Group III-V nitride compound semiconductor such as AlGaInN. Some of them have already been practically used.
Such blue-violet laser diodes (hereinafter referred to as “blue-violet LDs”) are formed by growing a compound semiconductor in crystal form on a GaN substrate.
A representative compound semiconductor is the Group III-V compound semiconductor, in which Group III and V elements are combined together. Mixed crystal III-V compound semiconductors having different compositions can be formed by bonding pluralities of Group III atoms and Group V atoms in different manners. Examples of compound semiconductors used to form a blue-violet LD include GaN, GaPN, GaNAs, InGaN, and AlGaN.
In ridge waveguide LDs, an electrode layer is usually provided on top of the waveguide ridge. This electrode layer is connected to the contact layer (i.e., the top layer of the waveguide ridge) through an opening formed in the insulating film covering the top portion of the waveguide ridge. This insulating film with the above opening is formed by lift-off using the same resist mask that was used to form the waveguide ridge. However, since the surface of the resist mask in contact with the contact layer is concavely curved with respect to the surface of the contact layer, part of the material used to form the insulating film covering the waveguide ridge remains in this concave portion and hence partly covers the surface of the contact layer even after the lift-off process, resulting in a reduction in the contact area between the electrode layer and the contact layer. The contact area is smaller than the top surface area of the contact layer.
In the case of a red LD, the material used to form the contact layer (e.g., GaAs, etc.) has a relatively low contact resistance. Therefore, the above reduction in the contact area between the electrode layer and the contact layer due to the lift-off process does not significantly increase the contact resistance, and hence the operating voltage of the LD.
In a blue-violet LD, on the other hand, the material used to form the contact layer is GaN, etc. having a relatively high contact resistance. Therefore, a reduction in the contact area between the electrode and the contract layer results in an increase in the contact resistance between them, thereby increasing the operating voltage of the blue-violet LD.
There will now be described several known methods for manufacturing an LD in such way as to prevent a reduction in the contact area between the electrode and the contact layer.
A first method forms a nitride semiconductor laser device in the following manner. First, a p-type electrode layer of palladium/molybdenum/gold is formed on a p-type contact layer which is the top layer of the semiconductor layer stack formed on a wafer. A resist mask (not shown) having a stripe shape is then formed on the p-type electrode layer and used to form a ridge stripe by RIE (Reactive Ion Etching). More specifically, the p-type electrode is formed by etching using Ar gas, and then the p-type contact layer and the p-type cladding layer, or these layers and the p-type guiding layer, are etched by a mixed gas composed of Ar, Cl2, and SiCl4 to form the ridge stripe. The etching is stopped at a depth halfway through the p-type cladding layer or the p-type guiding layer. Next, an insulating film (of Zr oxide predominantly including ZrO2) having a thickness of 0.5 μm is formed over the surface of the wafer with the ridge strip still leaving the resist thereon. The resist is then removed to expose the top surface of the ridge stripe. Further, a p-type pad electrode of molybdenum and gold is formed to cover the p-type electrode and at least the portions of the insulating film on both sides of the p-type electrode. (See, e.g., Japanese Domestic Republication of International Patent Application No. WO 2003/085790, lines 42-50 on page 9, and FIG. 1.)
A second known method is a self-aligning method for manufacturing a ridge waveguide semiconductor LD and includes the step of forming two different photoresist layers one on top of the other, as described below.
The lower photoresist layer is sensitive only to light of wavelengths shorter than 300 nm, while the upper photoresist layer is sensitive only to light of wavelengths longer than 300 nm. Specifically, this self-aligning method is applied to a laminated semiconductor structure that includes a second cladding waveguide layer and a capping layer formed on the second cladding waveguide layer. The method begins by removing portions of the capping layer and the second cladding waveguide layer to form a ridge structure and a double channel. A second insulating film is then formed on the surfaces of the ridge structure and the double channel. A first photoresist layer, the lower photoresist layer, is then formed on the second insulating film, and a second photoresist layer, the upper photoresist layer, is formed on the first photoresist layer. Next, the second photoresist layer is patterned to expose the portions of the first photoresist layer around the ridge structure. Further, the first photoresist layer is processed by an RIE process to expose the portion of the second insulating film on the ridge structure. The portions of the second insulating film around the ridge structure are then removed by an etching process including an RIE process. The remaining portions of the first and second photoresist layers are then removed, and a first metal layer is deposited as an electrode. (See, e.g., JP-A-2000-22261, paragraphs 0024 to 0034, and FIGS. 7 to 18.)
A third known method first forms a ridge and channels by wet etching the contact layer by using a metal mask (of Al) and then wet etching the underlying layer by using as a mask this contact layer with the metal mask still thereon. Next, an insulating film is formed over the entire surface of the substrate by plasma CVD, and the Al pattern (the metal mask) and the portions of the insulating film on the Al pattern are removed by lift-off. A resist pattern is then formed by a common lithographic process. This resist pattern exposes the portion of the surface where a p-type electrode is subsequently formed. A layer of electrode material is then formed by vacuum deposition using the resist pattern as a mask, and subsequently the resist pattern and the portion of the electrode material layer on the resist pattern are removed by lift-off to form the electrode in close contact with the contact layer of the ridge. See, e.g., JP-A-2000-340880, paragraphs 0025 to 0034, and FIG. 1.
A fourth known method proceeds as follows. A first protective film is formed over substantially the entire surface of a contact layer, and a third protective film having a stripe shape is formed on the first protective film. Then, after etching the first protective film with the third protective film thereon, the third protective film is removed. This step processes the first protective film into a stripe shape. The p-side contact layer is then etched through its entire thickness and the underlying layer (for example, a p-side cladding layer) is etched halfway through its thickness to form a waveguide having a stripe shape. Next, a second protective film, which has electrically insulating properties and is made of a different material than the first protective film, is formed on the sides of the stripe-shaped waveguide and on the top surface of the nitride semiconductor layer (i.e., the p-side cladding layer) exposed by the above etching process. The first protective film is then removed by lift-off, and a p-electrode is formed on the second protective film and the p-side contact layer such that the p-electrode is electrically connected to the p-side contact layer. (See, e.g., JP-A-2003-142769, paragraphs 0020 to 0027, and FIG. 1.)
The above conventional methods provide a sufficient contact area between the contact layer of the waveguide ridge and the electrode layer. However, these methods are disadvantageous in that it is difficult to reliably manufacture devices having substantially equal characteristics, since they include the step of: etching a metal film and the underlying semiconductor layer at the same time; etching the lower of two laminated resist layers to a predetermined controlled depth; or forming an electrode by lift-off after forming a metal film mask or a plurality of protective films. Further, employing a plurality of resists or protective films results in reduced freedom in process design.